Disposal of sludge from electrolytic preparation of sodium



Aug. 18, 1953 B. E. HODGE 2,649,413

DISPOSAL OF SLUDGE FROM ELECTROLYTIC PREPARATION OF SODIUM Filed July 19, 1950 |4-DISCHARGE LINE Z'FEED CHUTE k7 REACTOR JET PUMP AND SCRUBBER INVENTOR.

BENJAMIN E HODGE Patented Aug. 18, 1953 to Ethyl- Corporatipn, New York, N. Y., a. corporation-f Delaware Afiiilititt'i'diimily'm, 1950; SeriaINb; 174,655 watts; (Cl.'2-521 This invention relates toreactionsof alkaliand alkaline earth metals:- More specificallm-thewin vention relates to the dxidationpf amixture of an alkali and alkalineearth metal; suchas a mixture predominating sodiiimand calcium, and the simultaneous-further-reactioii ofthe oxidiZed alkali metal with 93 siliceous niaiteli'all In the manufactur'eoftlie alkali ni'et'alsby' elec trolysis of fused salts}byproduct mixtures con taining appreciable amouflts'=-'ofthe alkali metal are normally ericou-r'ite'rdi In addition to the alkalimetal such byproduct streams contain an alkaline earth metal plusminor quantitiesof impurities comprisingprincipally" the ahd chlorides of the n'ie'tal components; These' by product mixtures are the result of partial elec trclysis of components of theeiectroiys s-cen bath which are addedto-lowefthe ifielting point 'of th'e mixture; 7 A

A typical mixture inthe maniifacture-o'f sodium by the electrolysis ofsodi m' clilo'ride is th'e'b'y product mixture of sodi -cammnL-andminor amounts of sodium-and'calciifirl'oxidesi- 'Ihe'calcium meta-1 occurs as'the result of thef eleetrolysis of a small amount of -tlie calci-umchloridefwhich is component'of the-'electrolysis"'cell bath or electrolyte; The product from th'e cells, contains onlyslight amounts of calcium metal, but 'subselquent purification --steps-"r e'su'lt ina separate byproduct streamcontang rrom 10 w 3oweigia percent metallic calcium? H a y The disposal of; or-util izati'on f-- his mixture, has long been a problenrirf theindustry: is wellk'nown, sodium andthefotheralkali metais are very reactive andmust-beprocessecfwitlrcaution to avoid accidents} In p'articular'; exposure to water is avoided, because of the -violenceof the ensuing reaction. Many ttempts'i mve been made to successfully 'recove th 'diu m content of sodium-calcium byproducts. F r examplerecycling to the electrolysiscellsfhas ficfistdi'rlary. This procedure is' disadvahtageous;f'because the partial recovery of sodium' cohtntiSmore than offset by'the loss in 'production 'ofsodium which would normally beachieved from'such'cells and the power consumedtherein l I An object of my invention istoprovideamew and improved method for-"converting the alkali metals and mixtures thereofwithialkalineearth metals to disposable or safely solublei'products.

Another object is w mvme'a process rbr converting sodium-calcium mixturee withoutf the necessity of electrically treatin'g' h niaterials. A more specific object is to pro process for completely oxidizing alkali alkahne' ea'rtlr "metal chute Z, fitted with a, removable cover 3 is at-.- 45" 2 mixtures at a high rate and without the necessity of expensive refractory materials of construction. An additional specific object is to provide a selfsustaining and continuous process for converting alkali metal and alkaline earth metal mixtures to disposable or usable products.

The method" of my invention comprises oxidizing the alkali and alkaline earth metals with air or other gaseous oxidizing agent, and simultaneously reacting the alkali metal oxides so produced with a granular siliceous material and forming alkali metal silicates. According to my invent'ion', sand'or similar granular, siliceous material is fedconcurrently with the alkali metal to a reaction vessel. Air is contacted with the reactant solidsfed and'the metals present are oxidized to the alkali metal monoxide and alkaline earth metal oxide respectively, releasing sufiicient heat to promote the'reaction of the alkali metal oxide with'the' siliceous material. The siliceous material is'fed in proportions at least s'uflicientj to react with all the alkali metal monoxide obtainable fronf-the' alkali metal content of the feed material. but preferably not in such great quantity that the reaction is prevented from being continuous and self-sustaining.

The method of the invention will be more easily understood from the accompanying figure in conjunction with the working example hereafter given. The figure shows diagrammatically appaapparatus for inducing flow of gaseous oxidizing 35 agentand-jalso-for removing entrained solids from the; gases discharged. In the embodiment illustratedby thefigure, the auxiliary apparatus comprisesa jet pump which acts in the dual capacity of inducing gas flow and also of fully removing entrainedsolid products of reaction from the exit gas.

Referring to the f gure, a rotating cylinder or kilnl is thereaction chamber for the process. A

tached to a stationary end plate 4 and projects slightlyinto the interior oi kiln I. The discharge endof the kiln is. covered with a stationarycover lat 5;, m w i h a di c r c opzfirf c nt t i asho d ta e. A fl ct p ate or chute l-attachedto the exterior of cover plate 5 guides the discharged solid products into a portable 'bin -8. g

Air flowthrough the kiln'is'induced by; and the .exit gases-are' concurrently-scrubbed free of solid particle's by a' combinedjet 'pump or aspirator 3 and scrubber 9. The jet pump is a Venturi type aspirator utilizing a change in static head accompanying a change in velocity head, of an activating or motivating fluid, to induce flow of another fluid into the motivating fluid channel. A motivating fluid is introduced through valve H and line 12 into jet pump 9, the motivating fluid being preferably steam although compressed air may be used. Movement of the steam or other motivat ing fluid through jet pump 9 induces the required air flow, entering the kiln I through discharge chute 6. Gases leaving the kiln pass through the feed chute 2 into discharge line M to the jet pump 9. Vv'ater is supplied to the jet pump 9 through line l and valve l3 for scrubbing the gases and removing therefrom solid particles, in particular,

minute particles or dusts of sodium oxide,

The following working example is an embodi- J ment of the invention in the above described apparatus.

Example A byproduct mixture resulting from the manu- 4 or other alkaline earth metals such as strontium or barium. In the case of sodium-calcium mixtures, the relative proportions of the components varies appreciably. In general, the mixture contains from 60 to 70 percent sodium, and from to percent calcium metal content.

The specific conditions of the process can be varied widely without departing from the principles of the invention. An important variable is the amount. of sand or other siliceous material 'which is fed with the alkali metal feed. The

general requirements in this regard are that the siliccousmaterialmus't be in such proportion as to supply atleast one mole of silica per mole of alkali metal fed. This assures that the alkali metal content can be fully converted to a disfacture of sodium was fed to kiln I, the mixture 7 having the following composition:

Weight percent This mixture was fed through feed chute 2 concurrently with sand, the sand being fed at the Sodium 69 Calcium 23 Sodium monoxide 6 Calcium oxide 2 rate of 3 parts by weight to one of the sodium 7 containing mixture. Air was passed through the Weight percent Sodium silicate 34 M Silica Calcium oxide 5 Sodium monoxide 1 The oxidation in the kiln resulted in the formation of a smoke or fog of finely divided particles of sodium monoxide, which was removed from the gas stream by the jet pump-scrubber 9 and converted into a solution of caustic soda by dissolvin in the scrubbing water. Approximately 12 percent of the input sodium metal was converted to sodium oxide and discharged in this manner,

resulting in 14 pounds of caustic soda per 100 pounds of initial sodium-calcium mixture.

The solid product, of the composition given above, was a granular and free flowing material.

'The soluble compounds therein could be easily and safely leached with water and the resultant solution discarded. A suitable and economical use, however, of the granular material would be as a neutralizing agent for acidic wastes. If desired, the siliceous residue remaining after leaching, or after use as a neutralization agent, could be drained of water content and reused in processing additional alkali metal feed mixture.

It will be understood that, though the predominant or usual application of the invention is in connection with the reaction of sodium-calcium byproduct mixtures, the process is equally suitable for processing mixtures containing other alkali metals, for example, potassium or lithium,

posable product which is substantially free of alkali -metal as such. On the other hand, the proportion of siliceous material fed should'preferably not be so great that the process is no longer self sustaining thereby requiring the use of an external supply of heat.

In addition to the necessity of providing sufficient .silica to allow full conversion of the alkali metals to. the desired product, it has been discovered that the proportion of siliceous material is particularly important with respect to inhibiting or substantially reducing corrosion of the metal of construction of the kiln or reaction chamber. It has been further discovered that excessive corrosion is avoided by utilizin sunficient siliceous material to provide a quantity of silica which exceeds a variable lower limit. This lower limit of the preferred range has been found to be largely a function of the free alkali metal concentration in the original mixture of alkali and alkaline earth metals. Thus, for metal feed mixtures containing relatively low concentrations of sodium, for example, lower ratios of sand to sodium arepermissible than when a high proportion of sodium is initially present.

It has been found that the lower limit of the preferred range of moles of silica mole or atom of sodium is 2.3 times the original weight fraction of sodium present. Thus, when the original metal mixture contains weight percent sodium, the

.minimum ratio of silica to sodium will be 1.38

moles per atom or mole of sodium. As 100 pounds of metal mixture fed contains 2.6 pound atoms of .ti'ons. and calcium, for example, it has been found that whichever is the lowest ratio. ter of the upper limit of the preferred range is sodium, then the siliceous material fed should provide at least 3.6 pound moles of silica per 100 pounds of metal mixture. For sand of 98 percent silica content, then, the feed ratio of sand to metalsmixture would be preferably at least 220 pounds per 100 pounds of metals mixture fed.

With respect to the upper limit of the preferred range of sandto the metals mixture feed, this ratio is of importance with respect to the heat balance of the process. The quantity of sand, for greatest efliciency, should not be so great that the heat evolved by the reaction is insufficient to maintain the entire mixture at reacting condi- In processing a feed mixture of sodium the upper extent of the preferred range of sand vto metals mixture is 5:1, or sand equivalent to providing.2.8'moles of silica per mole of sodium,

The dual characnot fully understood but is apparently the result of the heat effects of the several reactions carried out in the process;

As an illustration of the operation of the preferred' upper 1imit,1in converting a feed mixture containing 23 percent sodium, the preferred upper limit is sufficient sand to provide 170 pounds of silica per 100 pounds of the metal mixture fed. On the other hand, if the feed mixture contains 7 0 percent or more sodium, the preferred quantity of sand will not exceed five parts to one part of metal mixture feed.

The preferred ranges will vary slightly in processing mixtures of the different alkali metals. Thus, if the alkali metal is potassium, the upper limit of the preferred range will usually be a sandzmetals weight ratio of 3:1.

A surprising feature of the process is that the siliceous material fed is not. necessarily bone dry, but can contain appreciable amounts of water. Thus, wet sand containing 3 percent water, has been safely and satisfactorily used. The only apparent limitation in this regard is that the water should not be admitted as, a separate stream. The practical importance of this finding is that no predrying of the siliceous material is required, and that the sand used can be stored in the open.

With respect to the particle size of the siliceous material, it is not essential that a narrow screened fraction be used. However, for most eificient use, it is preferred that the material will entirely pass an 8 mesh screen. Ordinary sand is the preferred silica source, although other comminuted siliceous material is suitable.

As a result of my invention, it is now possible to safely and completely convert all the metal content of alkali-alkaline. earth metal mixtures into easily disposable or useful. product. The predominant product is a. granular, free flowing, solids mixture discharged as such from the reaction chamber. However, an appreciable portion of the alkali metal is converted to a fine, powdery form of the corresponding monoxide. This material is so light and finely divided that the air flow necessary for thereaction invariably sweeps the powder from the reaction vessel. The monoxides of the alkali metals, in this powdered form, are irritating to the skin and, constitute also a respiratory nuisance to the workmen. The removal of such dusts from the exhaust gases is therefore vital to the success of the process. It has been found that these alkali metal oxide particles predominate in particles of less than one micron size; over 90 percent being below two microns size. Such particles are difficult to remove from gas streams by conventional scrubbing methods. A Venturi scrubber or jet pump does, however, make possible the efficient and complete removal of sodium oxide in this form, in addition to inducing. the required flow of oxidizing gas. This type of pump utilizes steam or other motivating fluid to, create asuction. In the present invention, a supply of water or an aqueous solution is also admitted to the throat of the venturi under the influence of the negative pressure created by the motivating fluid. The water is intimately mixed with the sodium oxide fog or smoke particles present in the gas from the reactor and are dissolved therein, thereby allowing venting of the cleaned or scrubbed gas to the atmosphere. When desired, the aqueous caustic solution formed in the scrubber can be recycled for absorption of additional sodium oxide to produce caustic solution of required strength.

The process can be carried out in many types of reactors, providing that the charge is uniformly agitated during the reaction. .As examples of reactors which can be suitably employed, furnaces of the Herreshof type, or single hearth furnaces with mechanical rabbling devices are suitable. The preferred reactor, however, is a rotating cylinder orkiln type reactor, as heretofore described. It has been found that a rotating kiln is the most economical reactor for the purpose and has a surprisingly large capacity. As a general rule, a kiln type reactor will process a mixture containing about 40 pounds of sodium per hour per square foot of cross sectional area. It is preferred that the kiln should be very nearly in a. horizontal position in order to provide adequate reaction time, the preferred reaction time being seven to ten minutes. A tilt of about onehalf a degree from the horizontal is a suitable kiln inclination. In general, peripheral speeds of kiln type reactors in the order of to feet per minute are preferred. 7

An important practical advantage of the process is that it isnow possible thereby to fully oxidize and react mixtures of alkali and alkaline earth metals in a reactor constructed exclusively of metal. It has previously been considered necessary to carry out the oxidation of alkali metal containing wastes by batch operations in large reaction chambers or rooms. According to that procedure, manual agitation is required for control of the reaction. This prior method is, of course, unsuitable for the continuous processing of metal mixtures containing high proportions of alkali metals. In addition, it has heretofore been considered essential that a refractory lining be utilized in a reaction chamber to convert such metal mixtures by an oxidation reaction. By the present process, however, large quantities of material can be continuously converted, and refractory materials of construction are not essential. The siliceous material introduced as a reactant in the process has been discovered to exert a pronounced inhibiting effect with respect to corrosion of metals in contact with the reacting mixture. A reduction in the corrosion of from 50 to 90 percent is achieved from the rate encountered without the siliceous reactant.

As an example of the pronounced benefits realized in minimizing corrosion, in oxidizing a typical sodium-calcium mixture in contact with Monel metal, corrosion at the rate of over one inch per year is encountered. In converting the same original mixture according to the present process, the corrosion rate is reduced to approximately one-tenth inch per year. The effectiveness of the siliceous material in inhibiting corrosion is apparently a dual phenomenon resulting from the combined mechanical dilution of the reacting materials, plus the conversion of the alkali metal component to high melting friable silicate material. In the absence of siliceous material, a mixture of alkali and alkaline earth metal oxides exists in the reactor forming a pasty mass which adheres to the reactor surfaces and tends to concentrate reaction at the reactor walls, thereby resulting in the high rate of corrosion heretofore described. Although mechanical dilution and heat absorption is one result of the use of siliceous material according to the process, mere mechanical dilution with inert material is not enough to accomplish the objects of the process. Thus, in attempts to obtain the desired results with mechanical dilution above, a supply of oyster shells (a convenient form of calcium carbonate) was jointly fed with a sodium-calcium feed mixture. This expedient was not entirely successful, in that the corrosion rate was not reduced to the degree possible, and the product obtained was not a free flowing usable material.

The preferred metal of construction is nickel or nickel clad steel; Other corrosion resistant alloys are also suitable, for example, Monel metal. If desired, a refractory lining can be employed in the reaction chamber although its use is not essential as heretofore explained. The use of corrosion resistant alloy or refractory lining is advantageous in obtaining the maximum service life of the equipment. Such increased service life can be obtained even if only a part of the reactor is so constructed, as explained below,

The operating temperatures within the kiln are maintained in the range of 1600 to 1900 F. Higher temperatures in this range are encountered at the feed end, where the amount of unconverted alkali metal is greatest. At the discharge end, the reactive components being substantially all converted to oxides or silicates, the corrosive efiect is greatly diminished, and ordinary mild steel can satisfactorily be employed. In general, about one half of a rotary kiln reactor should be constructed of nickel or equivalent non-ferrous metal. In other forms of reactors, the same proportions should be used, that is, about one half of the reactor body of surface first exposed to the reacting materials should be made of nickel or an equivalent corrosion resistant material.

The flow of air to the process is maintained sufficient to provide at least enough oxygen to convert all the metal content of the charge to the corresponding oxides, and preferably an excess of 30 to 50 percent is used. Countercurrent flow of the air and solid reactants is preferred, this method lending itself to the simplest form of reactor design. Cocurrent flow is quite satisfactory for the process, but requires a discharge means for the solid products which will prevent the short-circuiting of air flow by intake at that point.

In start up of the process, a small amount of the metals mixture is introduced to the reactor at the feed point. The oxidation is started by igniting with a blow torch, and air flow and the continuous introduction of metals mixture and siliceous material is started. If desired the initial ignition can be accomplished by introduction of burning solid fuel or organic material. The feeds of the reactants can be by manual addition, for relatively small operations, in which case portions of siliceous material and metals mixture are alternately fed. For larger scale operations, it will be preferable to use continuous conveyers jointly feeding both sand and metals mixture to the reactor.

Having fully described and illustrated the manner of operating the invention, the following is what I desire to claim by Letters Patent:

1. A process for readily disposing of a sludge of sodium and calcium formed as a by-product from the electrolytic preparation of sodium, which process comprises heating the sludge with air and siliceous material containing unreacted silica, the siliceous materials containing at least one mole of unreacted silica per mole of sodium in said sludge, thereby causing the metals, unreacted silica and air to react and produce a readily disposable solids product mixture con sisting essentially of sodium silicate, calcium oxide and silica.

2. A process for readily disposing of a mixture of alkali metal and alkali earth metal having about 23 to percent alkali metal, which process comprises heating the mixture with air and siliceous material containing unreacted silica, the siliceous material containing at least one mole of unreacted silica per mole of alkali metal in said mixture, thereby causing the metals, unreacted silica and air to react and produce a readily disposable product mixture consisting essentially of alkali metal silicate, alkaline earth metal oxide, and silica.

3. The process of claim 1 in which the ratio of siliceous material to sludge is not greater than either live to one by weight or that which provides 2.8 moles of unreacted silica per mole of sodium, whichever is the lesser.

4. The process of claim 1 in which the reaction is carried out in a metal container and the ratio of siliceous material to sludge is at least that which provides a mole ratio of silica to sodium of 2.3 times the original weight fraction of sodium in the sludge.

5. The process of claim 1 in which the ratio of siliceous material to sludge is (a) at least that which provides a mole ratio of silica to sodium of 2.3 times the original weight fraction of sodium in the sludge, and (b) not greater than either five to one by weight or that which provides 2.8 moles of unreacted silica per mole of sodium whichever is the lesser.

6. The process of claim 5 further defined in that the siliceous material is sand.

BENJAMIN HODGE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 677,906 Van Denbergh July 9, 1901 1,685,520 Carveth Sept. 25, 1928 2,110,363 Howells et al Mar. 28, 1938 OTHER REFERENCES Mellor, vol. 2 Comprehensive Treatise on Inorganic and Theoretical Chemistry 1922, pgs. 470, Longman, Green & Co., N. Y. 

1. A PROCESS FOR READILY OISPOSING OF A SLUDGE OF SODIUM AND CALCIUM FORMED AS A BY-PRODUCT FROM THE ELECTROLYTIC PREPARATION OF SODIUM, WHICH PROCESS COMPRISES HEATING THE SLUDGE WITH AIR AND SILICEOUS MATERIAL CONTAINING UNREACTED SILICA, THE SILICEOUS MATERIALS CONTAINING AT LEAST ONE MOLE OF UNREACTED SILICA PER MOLE OF SODIUM IN SAID SLUDGE, THEREBY CAUSING THE METALS, UNREACTED SILICA AND AIR TO REACT AND PRODUCE A READILY DISPOSABLE SOLIDS PRODUCT MIXTURE CONSISTING ESSENTIALLY OF SODIUM SILICATE, CALCIUM OXIDE AND SILICA. 